This invention generally relates to mixtures resulting from the dimerization of norbornadiene. In particular, the invention relates to a mixture having a high concentration of a monoolefinic hexacyclic hydrocarbon known by the systematic chemical name of exo-exo stereoisomer of hexacyclo(7.2.1.0.sup.2,8 -.1.sup.3.7.1.sup.5.13.0.sup.4,6)tetradec-10-ene, (also designated as hexacyclo[9.2.1.0.sup.2,10.0.sup.3,8.0.sup.4,6.0.sup.5,9 ]tetradec-12-ene). The stereoisomer results from the catalytic dimerization of norbornadiene which is a C.sub.7 H.sub.8 bicyclic, diolefinic hydrocarbon. More particularly, the invention relates to a mixture of high concentrations of the exo-exo form of the hexacyclic dimer. The latter is a C.sub.14 H.sub.16, six-ring monoolefinic hydrocarbon. Also, the invention relates to a mixture of the foregoing which have been hydrogenated to convert the monoolefinic hexacyclics into completely saturated hexacyclics. Hydrogenation of monoolefinic hexacyclic dimer to the saturated dimer improves stability of the product towards oxidation thereby enhancing its utility as a high energy fuel. Completely saturated exo-exo hexacyclic dimer has utility as a component of high energy fuel.
An object of present invention is to provide a composition which has a maximum concentration of hexacyclic norbornadiene dimers and minimal concentration of pentacyclic norbornadiene dimers and other compounds. Also, the composition can be used as a component of a high energy fuel for use in either jet or rocket propulsion. Jet propulsion includes a jet engine which can be used for missile, plane and others and includes the three basic types, i.e., ramjet, turbo-jet and pulse jet. The term jet generally refers to a device requiring air whereas rocket generally refers to a device containing its own oxygen or oxidizing agent.
Another object of present invention is to provide a novel method for preparing the foregoing composition. Still another object is the dimerization of norbornadiene at both an excellent selectivity and conversion to the exo-exo form of the four possible stereoismeric hexacyclic dimers.
Norbornadiene is also known as bicyclo(2.2.1)-heptadiene-2,5. A method of preparation is disclosed in U.S. Pat. No. 2,875,256 issued Feb. 24, 1959. Norbornadiene is referred to as NBD hereinafter. NBD can be represented by either one of the following structural formulas: ##STR1##
Dimerization of NBD is disclosed in U.S. Pat. No. 3,377,398, issued Apr. 9, 1968. The disclosed process results in the production of various dimer mixtures. The process therein involves the use of an iron catalyst system, e.g., ferric acetylacetonate and triethylaluminum, and a temperature above 140.degree. C. The product of said method is a mixture which includes both the monoolefinic hexacyclic and diolefinic pentacyclic dimers. Said patent also disclosed that no dimerization occurs if the iron acetylacetonate of the catalyst system is replaced by cobalt acetylacetonate.
U.S. Pat. No. 3,282,663, issued Nov. 1, 1966, discloses the dimerization of NBD to both pentacyclic and hexacyclic dimers. In one example, tetrakis(triphenylphosphine)nickel is the catalyst, in another, iron acetylacetonate and triethylaluminum is the catalyst system. Use of cobalt acetylacetonte is suggested.
U.S. Pat. No. 3,326,992, issued June 20, 1967, discloses the partial hydrogenation of NBD dimer mixtures
U.S. Pat. No. 3,326,993, issued June 20, 1967, discloses the dimerization of NBD, in the presence of a certain cobalt-containing carbonyl catalyst, to heptacyclic dimers. The resulting dimer mixture contains major proportions of the completely saturated dimer.
U.S. Pat. No. 3,329,732, issued July 4, 1967, discloses an improved method for the dimerization of NBD. The catalyst comprises certain metal salts of the tetracarbonylcobaltate anion wherein the metal is zinc, cadmium, mercury or indium. Resulting dimer mixture contains predominantly hexacyclic dimers.
Catalytic reaction of NBD and butadiene is disclosed in an article in The Journal of Organic Chemistry, January, 1970, Vol. 35, title, "Catalytic Norbornadiene-Butadiene and Norbornadiene-1,1-Dimethylallene Codimerization", by A. Greco, et al., pages 271-274. One of the disclosed catalysts is a three component system of tris(acetylacetonate)iron-AlEt.sub.2 Cl-bis(di-phenylphosphino)ethane. AlEt.sub.2 Cl refers to diethylaluminum chloride. One of the dimers reported therein, i.e., Fig. 1e, has been identified as the exo-exo stereoisomer of the hexacyclic dimer of norbornadiene.
Also, a catalytic reaction of NBD is disclosed in an article in The Journal of the American Chemical Society, Vol. 94, July 26, 1972, starting page 5446, titled, "Dimerization and Trimerization of Norbornadiene by Soluble Rhodium Catalyst", by Nancy Acton et al.. This article discloses the exo-exo form of the hexacyclic dimer of NBD.
As the previous discussion indicates, many NBD dimers are possible. G. N. Schrauzer, in his review "On Transition Metal-Catalyzed Reactions of Norbornadiene and the Concept of a Complex Multicenter Processes" in Advances on Catalysis 18, 373 (1968) Acad. Press, describes the fourteen theoretically possible dimers of NBD. The possible dimers, grouped according to the number of their carbocyclic rings, are as shown in the accompanying drawing. Any and each of the dimers shown in the drawing have different physical and chemical properties.
The synthesis problem in the dimerization of NBD can be visualized from the number of possible isomers and that is to obtain both an excellent selectivity and conversion to a desired isomer.
The advantages of present invention are many. The production of the exo-exo hexacyclic form of the dimer is highly favored while the production of pentacyclics is strongly minimized. The latter are not desirable as high energy fuels because of their high melting points. Separation of pentacyclics from the hexacyclics is commercially not feasible. On the other hand, the exo-exo hexacyclic dimer can be easily separated from small amounts of unreacted feed and other products, particularly higher boiling polymers. Thus, a separated product can be obtained consisting essentially of the exo-exo material. The latter, after hydrogenation, provides a material which can be used as a component for high energy, high density fuel.